Some conventional sport boots of this type, such as cross-country ski boots designed for skating, comprise a collar whose rearward pivoting motion can be restricted by elastic means which counter this motion by virtue of a sliding linkage possessing limited spring movement and centered on the collar joint, by holding the collar in place on the front part of the upper when this collar is closed, or simply by stopping the collar on the heel of the upper. To ensure the wearer's comfort, a transverse slot, or notch in the shape of a "U" open at the top, is produced in the rear of the collar and on a portion of its upper edge, and a padding element which extends to a relatively degree beyond this edge is arranged on the inner surface of the collar. In this way, when the collar reaches its limiting rearward pivoting position or in the event of hyperextension of the leg, the posterior part of the lower leg, and in particular the Achilles tendon, may still bend backward by a determinate amplitude. This configuration prevents the impact effect that may occur between the posterior part of the lower leg and the top of the plastic collar. In these boots, the padding element is covered, in conventional fashion, by a woven and/or jersey fabric, this covering being most often differentiated into the outer, visible side of the boot and the inner side, which is in contact with the lower leg. In fact, differentiated parts are normally joined on the periphery of the padding and are conventionally connected by stitching which extends in a plane transverse to the vertical axis of the collar. The configurations imparted to the rear upper edge of the collar and padding are provided to avoid overly firm, abrupt contact between the posterior part of the lower leg and the peripheral edge of the collar, when the wearer's movement causes his foot to be extended, or even hyperextended.
However, these conventional configuration do not prove completely satisfactory, since there remain points and/or lines of contact at the spot where the padding element and the upper edge of the rigid plastic collar meet. Furthermore, when the padding element incorporates a sewn covering, the stitching itself impedes any potential deformation of the padding and forms a contact line which the wearer finds painful. Indeed, these problems arise mainly from the significant reduction of the contact surfaces between the posterior part of the lower leg and the collar, which occurs when the foot is extended at the same time that the collar is prevented from following this rearward motion. As a result, the contact surfaces decrease in size when the lower leg extends, in a rearward supported position, toward the stitching and/or at the point of junction between the padding element and the peripheral edge of the collar. Thus, for a given stress, a significant increase of pressure tends to become localized along these spots, which form lines of contact, the effect of which on the lower leg is equivalent to that of a string pulled taut.